1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
An image forming apparatus such as a copying machine comprises a photoreceptor drum, a charging device for charging the photoreceptor drum uniformly, an exposing device for exposing the photoreceptor drum to form an electrostatic latent image, a developing device for developing the electrostatic latent image, or the like. These components or a developer contained in the developing device undergo changes in characteristics with changes in the environment such as temperature and humidity, and changes in characteristics with time. The state of images formed by charging, exposing, and developing the photoreceptor drum fluctuates with the electric changes in characteristics. There has been known a technique of changing conditions of image formation at predetermined timing to suppress the fluctuation of formed images, which is so-called process control (refer to Japanese Unexamined Patent Publications JP-A 2003-91224 and JP-A 2005-352379).
The process control is normally executed in such a manner that an image of a test pattern is actually formed, a density of the formed image is measured, and conditions of image formation are changed to eliminate a difference in density between the measurement value and an ideal value. In addition, since the image forming apparatus in question can not be used while executing the process control, a user of the image forming apparatus may be forced to have inconvenience.
When the timing of executing the process control is too late, the state of the image formation is unstable, and when the timing of executing the process control is too early, a toner is consumed wastefully or job efficiency is lowered wastefully. Accordingly, it is important that the process control is executed at appropriate timing.
In recent years, with further colorization in image forming apparatuses, an image forming apparatus has a plurality of image forming sections. The requirement to printed images (print quality) is so high that the process control is executed at the time of turning the power on, or for each predetermined number of printed sheets, however, in view of the cost, the process control consumes more than a little of toner, and therefore the execution frequency thereof is desired to be reduced as much as possible.
In the JP-A 2003-91224, the timing of executing process control is determined based on a ratio of the number of sheets of monochrome print jobs to the number of sheets of color print jobs, so that the process control is executed at appropriate timing.
In the JP-A 2005-352379, in order to make the timing of executing process control appropriate, an area coverage of 3 to 30% is determined as text printing, and the number of sheets of print jobs to execute process control is switched between text printing and color printing.
However, a technique disclosed in the JP-A 2003-91224 is directed to obtaining a ratio of monochrome printing to color printing in an interval of predetermined number of sheets, and executing no process control for color printing when the color printing is few. Accordingly, the process control is executed for each predetermined number of sheets.
Further, the technique disclosed in the JP-A 2005-352379 is directed to differentiating conditions under which the process control is executed between text data and image data. However, also in this case, the process control is executed for each predetermined number of sheets.
As described above, since the timing of executing process control relates to the toner consumption, the job efficiency, and the print quality, it is desired to further optimize the timing.